The tryptophan (Trp)-derived plant secondary metabolites, including camalexin, 4-hydroxy-indole-3-carbonylnitrile, and indolic glucosinolate (IGS), show broad-spectrum antifungal activity. However, the distinct regulations of these metabolic pathways among different plant species in response to fungus infection are rarely studied. In this study, our results revealed that WRKY33 directly regulates IGS biosynthesis, notably the production of 4-methoxyindole-3-ylmethyl glucosinolate (4MI3G), conferring resistance to
Alternaria brassicicola, an important pathogen which causes black spot in
Brassica crops. WRKY33 directly activates the expression of
CYP81F2,
IGMT1, and
IGMT2 to drive side-chain modification of indole-3-ylmethyl glucosinolate (I3G) to 4MI3G, in both
Arabidopsis and Chinese kale (
Brassica oleracea var.
alboglabra Bailey). However, Chinese kale showed a more severe symptom than
Arabidopsis when infected by
Alternaria brassicicola. Comparative analyses of the origin and evolution of Trp metabolism indicate that the loss of camalexin biosynthesis in
Brassica crops during evolution might attenuate the resistance of crops to
Alternaria brassicicola. As a result, the IGS metabolic pathway mediated by WRKY33 becomes essential for Chinese kale to deter
Alternaria brassicicola. Our results highlight the differential regulation of Trp-derived camalexin and IGS biosynthetic pathways in plant immunity between
Arabidopsis and
Brassica crops.
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